Electrical submersible pumps (ESP) are used in the oil and gas production industry to pump production and other fluids upwards in the wellbore in deep water applications. Some large ESPs are generally capable of pumping from 10-40 k barrels per day depending on conditions and pump specifications. Typically, positioned above the ESP, or other submersible pump, is a standing valve which prevents back-flow (downhole) into the ESP when the pump is shut-off for maintenance, injection procedures, pressure tests, bleeding off produced gas, etc. The standing valve is closed by hydrostatic head when the pump is off, provided, for example, by production fluid in the tubing. For deep water applications, the standing valve can hold a substantial differential pressure across the valve to support hydrostatic head above the ESP and, when applicable, tubing pressure from operations above the ESP. The valve opens when the pump is turned on and produces a relatively low differential pressure drop across the valve to a designed pressure value. The valve remains open for production of hydrocarbons or pumping of fluids uphole.
An ESP may see hundreds of shut-offs each year, again depending on circumstances. It is important to prevent back-flow into most pumps, since such reverse flow tends to rotate the pump backwards, damaging the pump. The standing valve must be able to seal adequately against significant hydrostatic pressure, especially in deep water wells. For example, the standing valve may have to seal against about 8,000 feet of production fluid or hold 3500 psi differential pressure. The standing valve also must withstand the tubing pressure above the valve during uphole operations.
However, it is often desirable for the standing valve to allow leakage of fluid down into the pump to lubricate and prime the pump for when it is activated. Consequently, prior art tubing standing valves, are designed to open to downward fluid flow at a selected differential pressure. For example, the valve may open to a downward fluid differential pressure of 3600-4000 psi. Typically, the pressure differential sufficient to operate the standing valve to fluid flow downward is relatively higher than the differential pressure to open the valve to upward fluid flow.
These requirements, holding against a high pressure (hydrostatic and tubing), opening at a relatively low differential upward pressure for production, and opening at a relatively high pump-down pressure for leakage, make a workable design and manufacture more difficult. For example, relatively hard materials, such as tungsten carbide, and relatively exacting tolerances, for example, at the valve seat, may be required for the valve parts to adequately seal, provide a closely defined small cross-sectional differential area for opening, etc.
Commercially available pump-through fluid loss control devices are made by Halliburton Energy Services, Inc., and sold as PES® Pump-Through Fluid Loss Control Devices. A single valve is used for both production and pump-through, with biasing spring assemblies providing for a relatively lower differential pressure to open the valve for production and a relatively higher differential pressure (pump-down pressure) to open the valve for pump-through procedures. A movable valve element seals on a movable valve seat. Pressure from the ESP below acts on the lower side of the valve element, on a cross-sectional area radially inside the seat, to move the valve upward and allow production. The valve element is biased closed when insufficient pressure is provided from below. For pump-through of fluids downward, fluid pressure is built up in the tubing string above the valve, the differential pressure acting on a narrow annular cross-sectional area of the movable seat. The seat, which is at the upper end or as a shoulder of a slidable sleeve, for example, is biased upward by a biasing mechanism. Sufficient differential pressure moves the seat downward while the valve element is prevented from similar downward movement, thereby opening the valve assembly for downward fluid flow.
The invention disclosed herein is described largely in terms of a standing valve assembly for use above an ESP, for example, in deep water wells. However, such a standing valve assembly can be employed in combination with various tool and string configurations for various purposes, as those of skill in the art will recognize. For example, the tool assembly can be used to prevent the loss of completion fluids or kill fluids and to prevent contamination of the wellbore-perforated interval.